ScienceShot

Birds call fowl on cuckoo's antics

The common cuckoo is what biologists call a brood parasite; it lays eggs in other birds’ nests, hoping the mothers won’t notice. Often they don’t, because cuckoo eggs have evolved to closely resemble those of their victims. And once the cuckoo hatches, the invader seizes the nest, pushing out all the other eggs. The parasitized birds, however, are fighting back. There’s an ongoing evolutionary arms race between the cuckoo and its most frequent victims, such as the brambling and the red-backed shrike. The parasitized birds are evolving unique egg “signatures,” markings that help the mother distinguish an authentic egg from an impostor. Though scientists have had ideas about how this arms race works, a study published this week in Nature Communications suggests that the egg war isn’t playing out the way we’d once thought. The conventional wisdom is that recognizable eggs must have three visual qualities: They should be highly similar to eggs holding their siblings; distinct from eggs laid by other mothers of the same species; and have complex, dense markings to make them more difficult to mimic. But when researchers analyzed hundreds of eggs from eight different parasitized species, they found a different story. Eggs didn’t necessarily need all three attributes to be distinguishable. For example, brambling parents were able to identify their eggs even though there was a lot of variation among sibling eggs and little difference between eggs from different mothers. The scientists also found that complex, dense egg markings could actually make it more difficult for mother birds to recognize their eggs. The insights suggest moderation may be a winning tactic in the egg war: Markings need to be visually dense enough to convey some information, yet not so dense that the egg becomes unrecognizable.